A depolymerized natural rubber obtained by the depolymerization of a natural rubber is also called a liquid rubber. Because of its strong adhesivity and excellent crosslinking reactivity, the depolymerized natural rubber has found a wide application as a raw material for adhesives, sealants, calking compounds, and the like. It has also been used as a reactive plasticizer for improving the processing characteristics of solid rubbers such as compound rubber for tires.
Further, as compared to the case where a solid rubber is used, the processing procedure to produce rubber products can be eased by using a liquid rubber, and thus it is advantageous in view of energy consumption. For these reasons, a liquid rubber has been drawing considerable attention in various industrial applications.
However, since the prior art depolymerized natural rubber is made from a low purification natural rubber from which protein has not been sufficiently removed, it retains an odor characteristic of natural rubbers and causes coloration due to oxidation of the protein, etc. Accordingly, the prior art depolymerized natural rubber is disadvantageous in that it is seldom utilized as an adhesive for familiar products such as household articles and nursing articles or observable articles such as an adhesive for a photographic album which for such a purposed must be colorless and transparent. Moreover, the prior art depolymerized natural rubber may cause allergies induced by protein, which has recently become a problem, and thus such rubbers cannot be applied to products which when used are placed in contact with the human body.
In general, a depolymerized natural rubber can be obtained by subjecting a natural rubber to mastication, pyrolysis, photodecomposition, chemical decomposition or the like treatment. However, these treatments have the following disadvantages.
In the mastication process, a material rubber is subjected to processing by a roll mill or a closed mixer to break the rubber molecular chains by mechanical and thermal action. During this process, a loosening agent such as a mercaptan is added to the rubber to prevent the recombination of the rubber molecular chain thus broken, thereby accelerating the reduction of the molecular weight of the rubber. The loosening agent is an organic compound which gives a plasticity to the rubber in a small amount, thereby reducing the mastication time. This is also referred to as "mastication accelerator". This process can provide a depolymerized rubber having a uniform number-average molecular weight. However, the distribution of the molecular weight of the resulting depolymerized rubber is broad and the terminal structure of the resulting depolymerized rubber cannot readily be controlled.
The pyrolysis process is disadvantageous in that the reaction temperature is so high that not only decomposition but also crosslinking or recombination occurs, thus it is difficult to control the molecular weight distribution or the terminal structure.
In the photodecomposition process, a molecular chain is broken by the energy of light such as ultraviolet rays. However, peroxides or the like at ketone ends produced by breaking the main chain act as a sensitizer thereby causing sequential decomposition, hence it is difficult to control the terminal end, the molecular weight or its distribution. This process is also disadvantageous in that it facilitates an isomerization reaction that transforms cis-1,4 structure into trans-1,4 structure.
Known chemical decomposition processes include ozone degradation, oxidative degradation by an oxidizing agent such as hydrogen peroxide and phenylhydrazine, and oxidative degradation by a metallic ion catalyst. Ozone degradation should be effected at a low temperature. Further, since it is dangerous to use an ozone in a large amount as required in the degradation process, this process is used only as an analytical method and is not industrially valuable.
Production of a depolymerized natural rubber by oxidative decomposition with hydrogen peroxide or phenylhydrazine has been industrialized. In this regard, "Polymer Digest", page 90, October (1981) discloses a molecular weight-reduced natural rubber latex. It is reported that this rubber latex is a product of the oxidative depolymerization of a natural rubber latex with hydrogen peroxide. However, this product is not sufficiently liquefied but rather is a solid. Further, the ends of the molecular chain of this product are not substituted with functional groups. Similar examples can be seen in JP-A-58-152075 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") wherein it is disclosed that a depolymerized natural rubber is obtained by adding hydrogen peroxide to a natural rubber latex, and then allowing the mixture to react while blowing oxygen thereinto. The result is a solid rubber having a break strength of 3.5 kg.
On the other hand, "Rev. Gen. Caoutch Plast", vol. 61, No. 643, page 79 (1984) discloses a technique which comprises the depolymerization of a natural rubber latex with phenylhydrazine and air to obtain a depolymerized natural rubber having a molecular weight of from 3,000 to 20,000. As evident from "Makromol. Chem.", vol. 186, No. 12, page 2,441 (1985), such a depolyerized natural rubber is terminated by phenylhydrazine and thus possesses no reactivity which is necessary for the extension of the main chain. Further, it was reported in "Makromol. Chem. Rapid Commun.", vol. 7, No. 3, page 159 (1986) that a liquid rubber having a molecular weight of 10,000 or less and a molecular weight distribution as narrow as 1.6 or 1.7 as calculated in terms of Mw/Mn has been produced by adding hydrogen peroxide and methanol or tetrahydrofuran to a toluene solution of a natural rubber and then irradiating the resulting mixture with ultraviolet rays. However, it was later reported in "Makromol. Chem.", vol. 189, No. 3, page 523 (1988) that a re-examination of the process revealed that an intramolecular epoxide group had been erroneously assumed as the terminal hydroxyl group.
The present inventors' examination has also revealed that epoxidation and isomerization of the main chain occurred in the depolymerization reaction by irradiation with ultraviolet rays under the same conditions as above. Further, it was found that no functional groups such as a carbonyl group, a carboxyl group and a hydroxyl group were produced at the ends of the molecular chain. Moreover, the depolymerized natural rubber thus obtained had a molecular weight distribution as broad as 4 or more as calculated in terms of Mw/Mn (weight-average molecular weight/number-average molecular weight ratio), and the control thereof was difficult.
If the foregoing depolymerized natural rubber which is not terminated by functional groups at its molecular ends is crosslinked with sulfur or peroxide, crosslinking reactions occur in the molecule rather than at the molecular ends, thereby providing a crosslinked rubber having many free-terminal chains. Since these free-terminal chains do not contribute to the physical properties or rubber elasticity of the crosslinked rubber but impair the dynamic properties of the crosslinked rubber, such a crosslinked rubber is not useful for commercial products.
In order to overcome these disadvantages, it is necessary to use a liquid rubber terminated by functional groups at both molecular ends and carry out crosslinking with the use of a hardening agent having a functionality of 3 or more or to use a liquid rubber containing functional groups at three positions, i.e., at the both molecular ends and at an intramolecular position, and carry out crosslinking with the use of a bifunctional hardening agent to form a three-dimensional network.
In order to obtain a crosslinked rubber having a good rubber elasticity, it is preferred that its molecular weight distribution be as narrow as possible and its molecular chain be as flexible as possible, i.e., the depolymerized natural rubber must maintain the original microstructure of natural rubber as much as possible. This is because a depolymerized natural rubber can be applied for various uses on condition that it is able to exhibit physical properties equal to ordinary vulcanized rubbers by making it to have a three-dimensional network structure with the use of an appropriate hardening agent.